Abnormal Hypsochromic Shifts of Surface Plasmon Scattering by Atomic Ordering in Gold–Copper Intermetallic Nanoparticles

Abstract

Alloy nanoparticles are of importance to tailor their physical and chemical characteristics, which can overcome the limits of monometallic materials. In particular, the plasmonic properties of metal alloys are primarily tuned by varying their alloy composition and electronic structure. The alloys tend to be described as a linear combination of the constituent elements, and their plasmonic characteristics are also expected to follow Vegard’s rule. In this study, we demonstrated the importance of atomic ordering in Au–Cu alloy nanoparticles, which exhibited a rather opposite trend of photophysical properties against such rough estimation for random alloys. The intermetallic Au3Cu, AuCu, and AuCu3 nanoparticles with a diameter of 20 nm were synthesized using a modified co-reduction method under identical aqueous environments. The alloys were characterized by X-ray spectroscopic techniques precisely, confirming their compositions and atomic arrangements in crystals. Notably, plasmon light scattering on intermetallic nanoparticles collected by single-particle dark-field spectroscopy exhibited unexpected abnormal hypsochromic shifts with the Cu content increase, in contrast to linear bathochromic shifts of random alloys. Density functional theory calculations considered the combined effect of both intraband and interband transitions in the intermetallic compounds and successfully confirmed the hypsochromic shifts of the plasmon bands by atomic ordering in the Au–Cu alloys. Our findings suggest that atomic engineering in alloy nanoparticles, by modulating their intrinsic complex permittivities, provide a new way for regulating plasmonic optical properties and developing various alloy systems with desired photophysical properties.

Atomic-Scale Engineering of CuOx–Au Interfaces over AuCu Single-Nanoparticles

“…The AuCuO x -473 sample showed intense diffraction lines at 2-theta of 39.6°, 45.6°, 66.8°, and 80.4° and weak diffraction lines at 2-theta of 23.8°, 32.0°, and 52.4° for fct-AuCu. The intense diffraction lines appeared at 2-theta of 39.1°, 45.1°, 66.3°, and 79.8° on the AuCuO x -573 sample but the weak diffraction lines vanished, suggesting the partial phase-transition from fct to fcc due to the decreased Cu/Au ratio in the metal particle . The AuCuO x -673 and AuCuO x -773 samples displayed diffraction lines for the fcc phase, while the most intense (111) line shifted to 38.4° and 38.2°, showing the characteristic feature of fcc Au-rich particles.…”

“…The intense diffraction lines appeared at 2-theta of 39.1°, 45.1°, 66.3°, and 79.8°on the AuCuO x -573 sample but the weak diffraction lines vanished, suggesting the partial phase-transition from fct to fcc due to the decreased Cu/Au ratio in the metal particle. 33 The AuCuO x -673 and AuCuO x -773 samples displayed diffraction lines for the fcc phase, while the most intense (111) line shifted to 38.4°and 38.2°, showing the characteristic feature of fcc Au-rich particles. This result verified that Cu atoms in the fct AuCu particle segregated gradually and were oxidized into amorphous CuO x , leaving recrystallized Au particles in the fcc phase.…”

Accurate prediction of the optical properties of nanoalloys with both plasmonic and magnetic elements

“…Just a small fraction of studies so far considered compounds with lower chemical stability such as Al 20 , Ga 14 and Cu 21 , or alloys. In particular, the attributes of traditional plasmonic elements (Au, Ag) have been prevalently expanded by alloying with other coinage metals (Cu) 22 , noble metals (Pd, Pt) 23 – 25 , d-metals (Fe) 26 , 27 or sp -metals (Al) 28 . These examples pointed to alloying as a leading strategy to combine different physical and chemical properties at the nanoscale and achieve distinct phenomena with a potentially disruptive impact on the current landscape of photonic technologies 7 , 15 – 19 .…”